CAMP LEATHERNECK, Afghanistan 
Newly arrived U.S. forces in southern Afghanistan will target insurgents crossing into the country from Pakistan and be a "game changer" in a region long dominated by the Taliban, a top commander said Tuesday.

Col. George Amland spoke to journalists at Camp Leatherneck, a rapidly expanding base now home to around 7,000 U.S. Marines preparing to push deeper into Helmand province, an insurgent stronghold and a haven for violent criminals controlling a massive opium-poppy industry. Some 3,000 Marines are already deployed elsewhere in the province.

President Barack Obama has ordered 21,000 troops to Afghanistan this summer to beat back the Taliban eight years after the U.S.-led invasion and create the conditions needed for the Afghan government to extend its influence and allow foreign forces to return home.

Helmand borders Pakistan, where U.S. and European commanders say Taliban insurgents have enjoyed a safe haven in recent years. Washington has targeted insurgents there with missiles fired from unmanned drones and is trying to get Islamabad to take firmer action, believing it to be essential for success in Afghanistan.

The Marines' current area of operations is around 18,000 square kilometers (7,000 square miles), but they are not yet present in the border area.

Amland, the deputy commander of the 2nd Marine Expeditionary Brigade Afghanistan, said in the future the Marines and NATO forces "would address those traffic lines between Afghanistan and Pakistan."

Britain has several thousand troops in Helmand that have proved unable to stop the insurgency, and critics have predicted Obama's troop surge may be too small and too late to defeat the Taliban.

Amland disputed that prediction, saying the troop deployment was "an appreciable investment" that would provide a base for the Afghan government and security forces to build on.

"It is a very big game changer to have this many Marines in an area this size," said Amland.

The United States and its allies invaded Afghanistan in late 2001 because the country's extremist Taliban leaders were sheltering Osama bin Laden and al-Qaida, the Islamic terrorist group behind the Sept. 11 attacks.

The forces quickly defeated the Taliban, pushing the militants out of Kabul and their southern base in Kandahar. But a guerrilla war, which turned dangerously violent in 2006, has bedeviled the international coalition and Afghan government.

Amland said the Helmand insurgency was in many cases intertwined with the criminals who control the opium and heroin industry there and that officers were trying to work out exactly who to target.

"I wish it were as simple as looking at alleged Taliban leaders," he said. "We are going to have to assess what is really Taliban influence and what is a spin-off of the narco-industry and how these forces interact."

The U.S. surge will bring American troop levels from about 55,000 to more than 68,000 by the end of 2009 – about half of the nearly 140,000 U.S. troops currently in Iraq.

The buildup has led to comparisons with Iraq, where an influx of troops in 2007 is credited with helping to reduce violence. But unlike Iraq, where the U.S. plans to phase out its role by 2012, the military envisions a long-term presence in Afghanistan.